#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>
//Definition for singly - linked list.
struct ListNode {
    int val;
    struct ListNode* next;
};

struct ListNode* middleNode(struct ListNode* head) {
    struct ListNode* slow, * fast;
    slow = head;
    fast = head;
    while (fast != NULL&&fast->next!=NULL)
    {
        fast = fast->next->next;
        slow = slow->next;
    }
    return slow;
}
int main()
{
    struct ListNode* n1 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n2 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n3 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n4 = (struct ListNode*)malloc(sizeof(struct ListNode));
    struct ListNode* n5 = (struct ListNode*)malloc(sizeof(struct ListNode));
    n1->val = 1;
    n2->val = 2;
    n3->val = 3;
    n4->val = 4;
    n5->val = 5;
    n1->next = n2;
    n2->next = n3;
    n3->next = n4;
    n4->next = n5;
    n5->next = NULL;
    struct ListNode* head = middleNode(n1);
    return 0;
}